The post from Valve's Eric Hope reads the controller is currently going through, "Live playtests, with everyone from industry professionals to die-hard gamers to casual gamers," and because of this, "Realistically, we're now looking at a release window of 2015, not 2014."

Following those details, Hope wrote the following:

Obviously we're just as eager as you are to get a Steam Machine in your
hands. But our number one priority is making sure that when you do,
you'll be getting the best gaming experience possible. We hope you'll be
patient with us while we get there. Until then, we’ll continue to post
updates as we have more stories to share.

We've reached out to Valve for more information on the delay, and will update this story if we receive a reply.